strip_unapproved is an internal function which essentially 'deletes' layers
from the current ternary plot in the event that such layers are not one of the
approved layers. For a layer to be approved, it must use an approved geometry, and also an approved stat.
Refer to approved_layers for the current list of approved geometries and stats
strip_unapproved(layers)strip_unapproved returns a list of approved layers (may be empty if none are approved).
list of the layers to strip unnaproved layers from.